The oscillation (the +/- data of the plot) won't be tighter. If I could have adjusted the average offset on all those graphs to better match the dash MPG readout (putting the average offset through zero on the graph), the oscillation would still be the same, only the average error offset would have changed.
Why did I pick up 1.8mpg going to a higher viscosity oil ?
- Thread starter Camperguy
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My experience has been that around-town/city driving....dash is over. On the highway, dash is under.
We're missing each other. The delta real to dash should be closer, it will still bounce around, but the absolute variance should be less is what I was trying to describe...at least that's my hypothesis.
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Patman
Staff member
The one problem I have with hand calculated fuel economy is how do you know that you filled up the tank to the EXACT same point you did the previous fillup? Even if you go to the same pump and just stop when the nozzle clicks off the first time, that could still not be filling it to the same exact point. So just based on that alone you could be off by a few tenths of a gallon, skewing the accuracy of the calculation.
Yes, the delta between dash MPG and hand calculated MPG would be closer matched if looking at the average of the +/- "oscillations" in the dash MPG vs hand calculated MPG data. But those oscillations would not be "tighter" like you mentioned in post 80. When you adjust your MPG factor, it's just moving the average to better match the hand calculation, but the +/- oscillations will remain between the dash MPG and hand calculated MPG.
There could be some of that, but if the same person is always dong the fill-ups it helps bring some consistency to that factor. If you look at the amount of skew from +/- two tenths of a gallon on a the MPG over a tank that's 16-20 gallons, the factor is pretty small, like 1% or less. The bigger the tank, the less sensitive that factor will be.
100% a possible error...most (including me when I was collecting data in my graph) just try to normalize it by getting it at the same station/same pump/until it cuts off...."good enough for govt work" is what I say
Agreed.
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For some reason, our GM is WAY off, to the negative. The BMW is exact to about 0.1, and the Lexus is exact to about 0.3.
To me, this indicates that they way they are metering/measuring the fuel is different. Someone out there knows why and how it's done.
And my buddy said the same thing about his Fords, way off.
It all goes back to high school stats class--reliable, and valid.
In a way it's like asking why the "range" on a car is so inaccurate, despite car manufacturers consulting the world's top fortune tellers and card readers, with astrological horoscopes baked in to the algorithms.
The mystery of the original question will never be answered, nor will the Loch Ness monster ever sign autographs...
(one of our earliest lessons in our time, occurred at age 5 or so, being baseball fans and calculating batting averages--it was a lesson in applied math that maybe we more or less observed and did, without fully understanding until we were 8, maybe 9. Seems to be the same with mpgs in the modern era, difference being we had to calculate long hand, and we were kids)
Patman
Staff member
In the case of my Civic it has a very small tank of only 12.4 gallons, so that factor would be bigger. I haven't bothered to hand calculate the fuel economy in this car, but according to the on board readout I'm averaging 5.2L per 100km since my last oil change 9000km ago. That's over 45 MPG. Even if it's off by 5% (which I doubt it would be that much), I'm still very impressed with the fuel economy of this car.
I have hand calculated the fuel economy in my Corvette though, and sadly it appears that the on board readout is around 5-7% optimistic. That's a bummer because on my previous Corvette (a 2005) the readout appeared to be right on the money.
Some people have reported a faster idle speed as well as cooler operating temperatures.
My vehicles are older so I have to hand calculate MPG. I do it for every vehicle I own at every fill up. Same pump, same station 99% of the time.
Since using the Euro 5w-40's my fuel mileage has decreased ever so slightly in Winter and increased ever so slightly in the Summer so it's a wash. Basically the same MPG as with other oils I've used.
Since using the Euro 5w-40's my fuel mileage has decreased ever so slightly in Winter and increased ever so slightly in the Summer so it's a wash. Basically the same MPG as with other oils I've used.
Your Civic's on-dash MPG readout could be off too ... the only way to know is to compare it to the hand calculations for many tank fill-ups. As seen in my post 76, every vehicle is different, and all of mine have been optimistic at different avg percentages with the on-dash MPG reading.
How much +/- did you see on your Corvette between the on-dash MPG and the hand calculated MPG like I show in the plots in post 76?
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Patman
Staff member
I didn't figure it out enough times in a row to get a chart going like yours, I would just every once in a while check it for a tankful and compare it and it is in the range of being 5-7% optimistic every time. It's the only car I've owned that is this far off.
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